Efficient Landscape Mesh Optimization
A landscape mesh can be simplified, while retaining most of the shape, using a combination of tools.

The decimate modifier using the planar option
Triangulating the mesh in the edit mode


Example:
This is a mesh for a portion of Bryce Canyon formed from a quad mesh. It has about 21,000 vertices.

Applying the decimate modifier with the planar option selected and 3degrees, then applying removes about 1/2 of the vertices, leaving about 13,000 vertices.

After applying the modifier and entering edit mode, the resulting mesh has the flat regions automatically merged. A final step is to triangulate the mesh in edit mode.


Note, there are other tools in edit mode that can help simplify the mesh:

Select &gt; Sharp Edges to select edges with high angles
Mesh &gt; Clean Up &gt; Limited Dissolve to eliminate edges with low angles
Mesh &gt; Clean Up &gt; Degenerate Dissolve to eliminate long skinny facets